Notch activation and signaling in the tumor microenvironment of Dll4-overexpressing BL41 tumors. Persistent Dll4 overexpression in BL41 tumor tissues removed from the mice 6 weeks after injection: human Dll4 mRNA levels measured by real-time PCR (A) and human Dll4 protein (pink) detected by immunohistochemistry (B) and immunoblotting (C) with human-specific Dll4 antibodies; NICD1 and NICD4 were detected by immunoblotting (C). Mouse HEY2 (D) and VEGFR2 (E) mRNA levels measured by real-time PCR (mouse-specific probes) in tumor tissues of Dll4- and control-BL41 tumors. Mouse VEGFR2 mRNA levels are corrected for mouse CD31 mRNA levels. Bars represent the relative mean plus or minus SEM mRNA levels (n = 10 A,D,E). VEGFR2 (green) and CD31 (red) immunostaining in representative BL41 tumor sections (F).